9/11: Revisiting World Trade Center attack site, then and now

Before: Pedestrians flee the area of the World Trade Center as the south tower collapses following the terrorist attack on the New York landmark Tuesday, Sept. 11, 2001. (Suzanne Plunkett / Associated Press).After: The skyscraper known as 1 World Trade Center is under construction in lower Manhattan, Aug. 4, 2011, in New York. (Mark Lennihan / Associated Press)

Before: Firefighters work beneath the destroyed mullions, the vertical struts once on the faces of the outer walls of the World Trade Center towers, on Sept. 11, 2001. (Mark Lennihan / Associated Press)After: The tower known as Four World Trade Center is under construction in lower Manhattan, Aug. 4, 2011. (Mark Lennihan / Associated Press)

Before: A destroyed subway station near ground zero on the evening of Sept. 12, 2001. (Mark Lennihan / Associated Press)After: Pedestrians walk along Church Street in lower Manhattan, Aug. 4, 2011. Behind them is the tower known as Four World Trade Center. (Mark Lennihan / Associated Press)

Before: The World Trade Center destruction is shown in a photo taken Sept. 30, 2001. (Mark Lennihan / Associated Press)After: The tower known as Four World Trade Center is under construction in lower Manhattan, Aug. 9, 2011. (Mark Lennihan / Associated Press)